In cases where image data from an image pickup apparatus using an image sensor, such as, e.g., a CMOS sensor, is displayed on a display apparatus as a moving image, it is preferable to acquire the image data from the image sensor in synchronization with the timing of a frame display of an image in a display apparatus. When the image sensor and the display apparatus differ in frame period, a method is known in which a memory (frame buffer) and a memory control circuit are arranged inside the display apparatus or between the display apparatus and the image sensor to once store frame data (image data of one frame) and output the frame data at the timing of the frame display of the display apparatus, i.e., in synchronization with the display operation of the display apparatus (see, e.g., Patent Document 1).
This method, however, has the following problems. It is required to store image data of one frame to several frames in a memory. This not only requires a large-capacity memory, which increases the device configuration size, but also requires a complicated circuit, which increases the cost. Further, display delay occurs due to the time required for the writing or reading operation with respect to the memory (frame buffer).
Further, when acquiring the same image by a plurality of image sensors, although it is preferable to synchronize the image acquisition timings (frame timings) of the plurality of image sensors. However, it is difficult to do so in the aforementioned method.
Among image sensors, there exists an image sensor having a mode called external trigger mode other than an internal frame synchronous operation mode (VIDEO mode) with a fixed timing of the inside of the image sensor. In this external trigger mode, since a synchronous operation by the external trigger can be performed, the frame memory as described above becomes unnecessary.
In some image sensors, however, there exist an sensor not having an external trigger mode and an sensor having an external trigger mode but its function is limited (for example, it cannot perform overlap that simultaneously performs exposure and image output of the previous frame and therefore speeding up cannot be performed). In such a sensor, even if it is good in performance, the sensor should be used in the internal frame synchronous operation mode (VIDEO mode), and therefore it was necessary to resynchronize using a frame buffer and a memory control.
Further, as a method not using a frame buffer, conventionally, a method is known in which a frequency of an operation clock of an image sensor is determined and generated from a period of a frame synchronization signal on an external device side and the number of clocks required for one frame, i.e., an operation clock of the image sensor is set to a frequency corresponding to the frame period of the external device (see, e.g., Patent Document 2). In some image sensors, however, there exists a device requiring a clock input of a recommended frequency to obtain a favorable performance. For such an image sensor, it is impossible to apply such a method.